Measurement of stress and strain in various types of bodies is a critical task in numerous fields of engineering. Mechanical, civil, and aeronautical engineers, designers working in many areas and those in charge of monitoring long term status of structures and devices all require the ability to check theoretical calculations of stress or strain against actual measurement of these factors. Machinery, vehicles, buildings, structures, precision equipment, even certain geological structures are all examples of the types of bodies on which stress/strain measuring is used. Despite the existence of well known theories for calculation of stress or strain, measurement remains vital because even in recent decades, computational models of stress and strain have been shown to be incorrect by strain and stress factors which may even exceed the ultimate/design load (working load times the factor of safety) originally provided. In addition, even when such models are correct, decisions made at the time of manufacture or construction may alter the factor of safety in unforseen ways, occasionally with tragic consequences. Finally, during the life cycle of such bodies, fatigue and other alterations in the structure's composition, the history of use of the body or other changes may change the stress or strain of the body.
For purposes of this application, the word “stress” will be used to refer to either stress or strain. While stress may be thought of as the imposed load on a cross-sectional unit area of a body and strain may be considered to be the imposed longitudinal deformation on the body, stress and strain are related by the material's modulus of elasticity (Young's Modulus) and thus for a material of known properties, one may be derived from the other.
This fact is used in the typical stress/strain sensor. Traditional sensors are simply devices (such as wires) firmly attached to the body to be measured, so that elongation of the body results in elongation of the sensor. The elongation per unit length of the sensor (the strain) may then be related to the stress in the body. However such sensors typically require an electronic connection to the measuring apparatus being used to record the measured strain/stress. Wires may be used, or telemetry, but some sort of apparatus beyond the sensor itself is necessary in the prior art.
For example, measuring torque on a rotating shaft is a complex problem: the rotation of the shaft may make a physical contact impossible; other considerations such as space may make telemetry impossible. Numerous attempts have been made to solve this problem, and the broader problem of remote sensing of strain and stress.
Pending US Utility Patent Application 09/651,806 filed Aug. 30, 2000 by Biter et al, (also inventor of the present invention) and entitled “METHOD OF SENSING STRAIN IN A MATERIAL BY DRIVING AN EMBEDDED MAGNETOELASTIC FILM-COATED WIRE TO SATURATION” teaches a non-linear response in the sensor film-coated wire. However, it does not teach remote magnetic induction of a self-current in the sensor, nor remote sensing of the magnetic field internally generated by the sensor's self-current.
U.S. Pat. No. 3,902,167 issued to Aug. 26, 1975 to Lutes et al for “MAGNETIC THIN FILM SWITCH” and U.S. Pat. No. 4,065,757 issued Dec. 27, 1977 to Kardashian for “THIN FILM PLATED WIRE MAGNETIC SWITCH OF ADJUSTABLE THRESHOLD” (related patents to the same assignee) teach switches wherein a thin film coated wire may generate a pulse when its polarity is reversed in an increased magnetic field of opposite polarity. As the '167 reference states at column 1, lines 46 et seq: “There are important differences between a magnetic sensor and a magnetic switch . . . The output of the magnetic sensor changes continuously . . . the switch, on the other hand, has only two states . . . ” Thus switches may not serve as useful prior art to sensors and in addition, various structural features of these two references result. The magnetic field generation means and/or the pulse detector are not remote to the sensing device, there is no magnetic induction of a self-current because the magnetic field is externally applied, there is no discussion of driving the magnetic film to saturation, there is no discussion of measuring highly non-linear components of the response (the “pulses” are merely detected for switching), and there is no teaching of measurement of harmonic or other alternative frequency responses. In addition, the current applied is minuscule, and the wire used is copper-beryllium, having a resistance too high to allow useful spike measurement.
U.S. Pat. No. 5,297,439 issued Mar. 29, 1994 to Tyren et al for “MAGNETOELASTIC STRESS SENSOR”, discloses a magneto-elastic element (see FIGS. 5 and FIG. 6 of the '439 patent) having a complex structure and geometry. The system used in the '439 patent allows remote sensing of the resonance frequency created in the magneto-elastic element. However, the '439 patent does not disclose that a harmonic frequency of the basic resonance frequency may be sensed, nor does the '439 patent teach or suggest the use of a simplified sensor having a non-linear current-voltage relationship.
U.S. Pat. No. 5,952,762 issued Sep. 14, 1999 to Larsen et al for “SLIP RING AMPLIFIER” addresses this same area by placing a rotor having an amplifier onto the rotating body, as shown in FIG. 4 of the '762 patent. A stator is fixedly mounted around the rotor; leaf contacts on a slip ring physically provide an electrical connection between the sensor/amplifier mounted on the body/rotor and the stator ring surrounding them. This assemblage is obviously not always practical, given considerations such as space, and the presence of the rotor/stator/slip ring/leaf contact combination may substantially alter the rotation of the body being measured.
U.S. Pat. No. 5,493,921 issued Feb. 27, 1996 to Alasafi et al for “SENSOR FOR NON-CONTACT TORQUE MEASUREMENT ON A SHAFT AS WELL AS A MEASUREMENT LAYER FOR SUCH A SENSOR” teaches (see FIG. 2 of the '921 patent) a coil coaxial to a shaft (not shown in FIG. 2) which is across a narrow air gap from a magnetostrictive measurement layer. This is a fixed installation, once again requiring a large space, and severely impacting the geometry of the body being measured. It does not teach or suggest the use of stress sensors having non-linear current-voltage relationships.
U.S. Pat. No. 5,970,393 issued Oct. 19, 1999 to Khorrami et al for “INTEGRATED MICRO-STRIP ANTENNA APPARATUS AND A SYSTEM UTILIZING THE SAME FOR WIRELESS COMMUNICATIONS FOR SENSING AND ACTUATION PURPOSES” teaches the use of a radio frequency antenna which receives a radio signal, modulates it based upon a piezoelectric substrate and re-radiates it. The system is based upon telemetry.
U.S. Pat. No. 5,902,934 issued May 11, 1999 to Sprague et al for “PHASE MAGNITUDE SIGNAL DETECTOR” uses a phase magnitude technique for measurement of torque, using magnetostrictive materials: materials which change magnetic properties under stress (or change shape in differing magnetic environments). The '934 teaches away from examination of magnitude of voltage-current response in sensing coils and further teaches towards increasing linear response in torque measurement sensors.
U.S. Pat. No. 6,026,818 issued Feb. 22, 2000 to Blair et al for “TAG AND DETECTION DEVICE” comes from the field of medical technology and teaches use of a magnetic “tag” attached to surgical equipment. In the event that a tagged item is accidentally left in a surgical site, it can easily be detected by means of a hand held “interrogation ring.”
In many applications, contact between the sensing system and the body monitored is undesirable or impossible. This is the case with shafts rotating within confined spaces, especially when retrofit of the sensing system is necessary. In other applications such as bridges, long term health monitoring is needed, yet wear and damage on sensors may mount quickly if the sensor is properly placed to monitor structural elements in high traffic zones or high weather impact zones. In addition, known non-contact stress measurement systems suffer from a variety of weaknesses including weak signal strength, difficulty of sorting out the signal from noise, power requirements, and so on. For measuring of stress in bodies, it would be desirable to provide non-contact methods and apparatus which provide more accuracy, greater reliability, and allow minimization of equipment and space required.